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Micro‐magneto‐Raman scattering spectroscopy is made use of to probe magneto‐phonon resonances (MPR) in suspended mono‐ to pentalayer graphene. MPR correspond to avoided crossings between zone‐center optical phonons (G‐mode) and optically active inter‐Landau level (LL) transitions and provide a tool to perform LL spectroscopy at a fixed energy (≈197 meV) set by the G‐mode phonon. Using a single‐particle effective bilayer model, the velocity parameter associated with each MPR is readily extracted. A single velocity parameter slightly above the bulk graphite value suffices to fit all MPR for N ≥ 2 layer systems. In contrast, in monolayer graphene, it is found that the velocity parameter increases significantly from (1.23 ± 0.01) × 106 m s−1 up to (1.45 ± 0.02) × 106 m s−1 as the first to third optically active inter‐LL transitions couple to the G‐mode phonon. This result is understood as a signature of enhanced many‐body effects in unscreened graphene. [ABSTRACT FROM AUTHOR]

We probe electronic excitations betweenLandau levels in freestanding N-layer graphene overa broad energy range, with unprecedentedspectral and spatial resolution, using micro magneto-Raman scatteringspectroscopy. A characteristic evolution of electronic bands in upto five Bernal-stacked graphene layers is evidenced and shown to remarkablyfollow a simple theoretical approach, based on an effective bilayermodel. (N> 3)-layer graphenes appear as appealingcandidates in the quest for novel phenomena, particularly in the quantumHall effect regime. Our work paves the way toward minimally invasiveinvestigations of magneto-excitons in other emerging low-dimensionalsystems, with a spatial resolution down to 1 μm. [ABSTRACT FROM AUTHOR]

Abstract: We review the recent literature on the use of optical spectroscopy of semiconductor quantum dots in high magnetic fields. We address both self-assembled epitaxial dots and colloidal nanocrystal quantum dots, each of which has its own characteristic optical response. Combining simple theoretical models for quantum confinement with the effect of high magnetic fields we describe the basic optically allowed transitions expected for epitaxial and colloidal quantum dots. Within these models we discuss the effects of quantum confinement and orbital and spin Zeeman effects on the optical spectra, illustrated by experimental examples. Finally, effects of electron–electron and exchange interactions are addressed. [Copyright &y& Elsevier]

The aptitude of magneto-spectroscopic methods for studying the electronic and spin properties of semiconductor structures is demonstrated with a few examples of our recent work on two-dimensional electron gases and semiconductor quantum dots, on bulk GaAs and GaN, as well as on thin graphitic layers. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate the creation of defects in monolayer WSe2 via nanopillar imprinting and helium ion irradiation. Based on the first method, we realize atomically thin vertical tunneling light-emitting diodes based on WSe2 monolayers hosting quantum emitters at deterministically specified locations. We characterize these emitters by investigating the evolution of their emission spectra in external electric and magnetic fields, as well as by inducing electroluminescence at low temperatures. We identify qualitatively different types of quantum emitters and classify them according to the dominant electron-hole recombination paths, determined by the mechanisms of intervalley mixing occurring in fundamental conduction and/or valence subbands. [ABSTRACT FROM AUTHOR]

We review the different results obtained in the last decade in the field of Raman scattering of graphene‐based systems, with an applied magnetic field. Electronic properties of graphene‐based systems with an applied magnetic field are first described. The phonon response in magnetic field, the magneto‐phono resonance, is then introduced and described for graphene monolayer and multilayer, as well as for bulk graphite. Electronic Raman scattering are then discussed in the context of Landau level spectroscopy, of electron–phonon interaction and of electron–electron interaction. Copyright © 2017 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Refined infrared magnetotransmission experiments have been performed in magnetic fields B up to 35 T on a series of multilayer epitaxial graphene samples. Following the main optical transition involving the n = 0 Landau level (LL), we observe a new absorption transition increasing in intensity with magnetic fields B ≥ 26 T. Our analysis shows that this is a signature of the breaking of the SU(4) symmetry of the n = 0 LL. Using a quantitative model, we show that the only symmetry-breaking scheme consistent with our experiments is a charge density wave (CDW). [ABSTRACT FROM AUTHOR]

Optical response of crystalline solids is to a large extent driven by excitations that promote electrons among individual bands. This allows one to apply optical and magneto‐optical methods to determine experimentally the energy band gap —a fundamental property crucial to our understanding of any solid—with a great precision. Here it is shown that such conventional methods, applied with great success to many materials in the past, do not work in topological Dirac semimetals with a dispersive nodal line. There, the optically deduced band gap depends on how the magnetic field is oriented with respect to the crystal axes. Such highly unusual behavior is explained in terms of band‐gap renormalization driven by Lorentz boosts which results from the Lorentz‐covariant form of the Dirac Hamiltonian relevant for the nodal line at low energies. [ABSTRACT FROM AUTHOR]

We present a microscopic theory of the magnetic field dependence of the optical properties of II-VI semiconductor quantum dots containing a single magnetic (Mn) impurity. The single-particle electron and heavy-hole states are described exactly by two-dimensional harmonic oscillators in a magnetic field, the Mn ion is treated as a spin of an isoelectronic impurity, and the quantum dot anisotropy is included perturbatively. The electron-hole direct, short-, and long-range exchange electron-hole Coulomb interactions, as well as the short-range spin-spin contact exchange interaction of the electron and the hole with the magnetic impurity is included. The electron-hole-Mn states are expanded in a finite number of configurations controlled by the number of confined electronic quantum dot shells and the full interacting Hamiltonian is diagonalized numerically in this basis. The absorption and emission spectrum is predicted as a function of photon energy, magnetic field, number of confined shells, and anisotropy. It is shown that the magnetic-field-induced enhancement of the exchange interaction of the Mn spin with the exciton is largely canceled by increased electron-hole Coulomb interactions. The predicted weak magnetic field dependence of the spacing of emission lines agrees well with the results of the spin model at low magnetic fields but differs at higher magnetic fields. Correlations in the exciton-Mn complex are predicted to determine absorption spectra. [ABSTRACT FROM AUTHOR]

Binding energies of negative and positive trions in high magnetic fields are compared. Simultaneous inclusion of several Landau levels and quantum well subbands in exact numerical diagonalization allowed quantitative description of the coupling between in-plane dynamics (governed by interplay of cyclotron quantization and Coulomb interactions) and single-particle excitations in the normal direction. Symmetric and asymmetric GaAs quantum wells of different widths were considered, as well as the effect of a possible binding of trions by sparse ionized impurities nearby the quantum well. [ABSTRACT FROM AUTHOR]

Monolayers of transition metal dichalcogenides display a strong excitonic optical response. Additionally encapsulating the monolayer with hexagonal boron nitride allows to reach the limit of a purely homogeneously broadened exciton system. On such a MoSe2‐based system, ultrafast six‐wave mixing spectroscopy is performed and a novel destructive photon echo effect is found. This process manifests as a characteristic depression of the nonlinear signal dynamics when scanning the delay between the applied laser pulses. By theoretically describing the process within a local field model, an excellent agreement with the experiment is reached. An effective Bloch vector representation is developed and thereby it is demonstrated that the destructive photon echo stems from a destructive interference of successive repetitions of the heterodyning experiment. [ABSTRACT FROM AUTHOR]

Strong Coulomb correlations together with multi-valley electronic bands in the presence of spin-orbit interaction are at the heart of studies of the rich physics of excitons in monolayers of transition metal dichalcogenides (TMD). Those archetypes of two-dimensional systems promise a design of new optoelectronic devices. In intrinsic TMD monolayers the basic, intravalley excitons, are formed by a hole from the top of the valence band and an electron either from the lower or upper spin-orbit-split conduction band subbands: one of these excitons is optically active, the second one is dark, although possibly observed under special conditions. Here we demonstrate the s-series of Rydberg dark exciton states in tungsten diselenide monolayer, which appears in addition to a conventional bright exciton series in photoluminescence spectra measured in high in-plane magnetic fields. The comparison of energy ladders of bright and dark Rydberg excitons is shown to be a method to experimentally evaluate one of the missing band parameters in TMD monolayers: the amplitude of the spin-orbit splitting of the conduction band. Excitonic physics dominates the optical response of semiconductor monolayers but single particle band structure parameters are hard to probe experimentally. Here, spin-orbit splitting in the conduction band of monolayer WSe2 is revealed by the identification of the Rydberg series of dark excitons. [ABSTRACT FROM AUTHOR]

When serving as a protection tissue and/or inducing a periodic lateral modulation for/in atomically thin crystals, hexagonal boron nitride (hBN) has revolutionized the research on van der Waals heterostructures. By itself, hBN appears as an emergent wide-bandgap material, which, importantly, can be optically bright in the far-ultraviolet range and which frequently displays midgap defect-related centers of yet-unclear origin, but, interestingly, acting as single-photon emitters. Controlling the hBN doping is of particular interest in view of the possible practical use of this material. Here, we demonstrate that enriching hBN with carbon (C) activates an optical response of this material in the form of a series of well-defined resonances in visible and near-infrared regions, which appear in the luminescence spectra measured under below-bandgap excitation. Two, qualitatively different, C-related radiative centers are identified: One follows the Franck-Condon principle that describes transitions between two defect states with emission/annihilation of optical phonons, and the other shows atomic-like resonances characteristic of intradefect transitions. With a detailed characterization of the energy structure and emission dynamics of these radiative centers, we contribute to the development of controlled doping of hBN with midgap centers. [ABSTRACT FROM AUTHOR]

In semiconductor quantum-wire heterostructures, interface roughness leads to exciton localization and to a radiative decay rate much smaller than that expected for structures with flat interfaces. Here, we uncover the electronic and optical properties of the one-dimensional extended defects that form at the intersection between stacking faults and inversion domain boundaries in GaN nanowires. We show that they act as crystal-phase quantum wires, a novel one-dimensional quantum system with atomically flat interfaces. These quantum wires efficiently capture excitons whose radiative decay gives rise to an optical doublet at 3.36 eV at 4.2 K. The binding energy of excitons confined in crystal-phase quantum wires is measured to be more than twice larger than that of the bulk. As a result of their unprecedented interface quality, these crystal-phase quantum wires constitute a model system for the study of one-dimensional excitons. [ABSTRACT FROM AUTHOR]

We report on experimental investigations of an electrically driven WSe2 based light-emitting van der Waals heterostructure. We observe a threshold voltage for electroluminescence significantly lower than the corresponding single particle band gap of monolayer WSe2. This observation can be interpreted by considering the Coulomb interaction and a tunneling process involving excitons, well beyond the picture of independent charge carriers. An applied magnetic field reveals pronounced magneto-oscillations in the electroluminescence of the free exciton emission intensity with a 1/B periodicity. This effect is ascribed to a modulation of the tunneling probability resulting from the Landau quantization in the graphene electrodes. A sharp feature in the differential conductance indicates that the Fermi level is pinned and allows for an estimation of the acceptor binding energy. [ABSTRACT FROM AUTHOR]

By implementing four-wave mixing (FWM) microspectroscopy, we measure coherence and population dynamics of the exciton transitions in monolayers of MoSe2. We reveal their dephasing times T2 and radiative lifetime T1 in a subpicosecond (ps) range, approaching T2 = 2T1 and thus indicating radiatively limited dephasing at a temperature of 6 K. We elucidate the dephasing mechanisms by varying the temperature and by probing various locations on the flake exhibiting a different local disorder. At the nanosecond range, we observe the residual FWM produced by the incoherent excitons, which initially disperse toward the dark states but then relax back to the optically active states within the light cone. By introducing polarization-resolved excitation, we infer intervalley exciton dynamics, revealing an initial polarization degree of around 30%, constant during the initial subpicosecond decay, followed by the depolarization on a picosecond time scale. The FWM hyperspectral imaging reveals the doped and undoped areas of the sample, allowing us to investigate the neutral exciton, the charged one, or both transitions at the same time. In the latter, we observe the exciton-trion beating in the coherence evolution indicating their coherent coupling. [ABSTRACT FROM AUTHOR]

Graphene layers are known to stack in two stable configurations, namely, ABA or ABC stacking, with drastically distinct electronic properties. Unlike the ABA stacking, little has been done to experimentally investigate the electronic properties of ABC graphene multilayers. Here, we report on the first magneto optical study of a large ABC domain in a graphene multilayer flake, with ABC sequences exceeding 17 graphene sheets. ABC-stacked multilayers can be fingerprinted with a characteristic electronic Raman scattering response, which persists even at room temperatures. Tracing the magnetic field evolution of the inter Landau level excitations from this domain gives strong evidence for the existence of a dispersionless electronic band near the Fermi level, characteristic of such stacking. Our findings present a simple yet powerful approach to probe ABC stacking in graphene multilayer flakes, where this highly degenerated band appears as an appealing candidate to host strongly correlated states. [ABSTRACT FROM AUTHOR]

Semiconducting transition metal dichalcogenides (TMDCs) give rise to interesting new phenomena in external magnetic fields, such as valley Zeeman splitting and magnetic-field-induced valley polarization. These effects have been reported for monolayers (MLs) of the transition metal diselenides MoSe2 and WSe2 and, more recently, for disulfides MoS2 and WS2. Here, we present helicity-resolved magneto-photoluminescence and magneto-reflectance contrast measurements for MLs of the telluride member of the semiconducting TMDCs, 2H-MoTe2, in magnetic fields up to 29 T in Faraday geometry. Well-resolved valley Zeeman splittings for the neutral A and B excitons (XA0 and XB0) and the charged exciton X± are observed with effective g-factors of -4.6 ± 0.2, - 3.8 ± 0.6, and -4.5 ± 0.3, respectively. The magnetic field induced valley polarization of XA0 and X± reaches 78% and 36%, respectively, at a magnetic field of 29 T. [ABSTRACT FROM AUTHOR]

The Aharonov-Bohm effect in ring structures in the presence of electronic correlation and disorder is an open issue. We report novel oscillations of a strongly correlated exciton pair, similar to a Wigner molecule, in a single nanoquantum ring, where the emission energy changes abruptly at the transition magnetic field with a fractional oscillation period compared to that of the exciton, a so-called fractional optical Aharonov-Bohm oscillation. We have also observed modulated optical Aharonov-Bohm oscillations of an electron-hole pair and an anticrossing of the photoluminescence spectrum at the transition magnetic field, which are associated with disorder effects such as localization, built-in electric field, and impurities. [ABSTRACT FROM AUTHOR]

The energy spectrum of common two-dimensional electron gases consists of a harmonic (that is, equidistant) ladder of Landau levels, thus preventing the possibility of optically addressing individual transitions. In graphene, however, owing to its non-harmonic spectrum, individual levels can be addressed selectively. Here, we report a time-resolved experiment directly pumping discrete Landau levels in graphene. Energetically degenerate Landau-level transitions from n = −1 to n = 0 and from n = 0 to n = 1 are distinguished by applying circularly polarized THz light. An analysis based on a microscopic theory shows that the zeroth Landau level is actually depleted by strong Auger scattering, even though it is optically pumped at the same time. The surprisingly strong electron-electron interaction responsible for this effect is directly evidenced through a sign reversal of the pump-probe signal. [ABSTRACT FROM AUTHOR]

Semiconductingcarbon nanotubes (CNTs) provide an exceptional platformfor studying one-dimensional excitons (bound electron–holepairs), but the role of defects and quenching centers in controllingemission remains controversial. Here we show that, by wrapping theCNT in a polymer sheath and cooling to 4.2 K, ultranarrow photoluminescence(PL) emission line widths below 80 μeV can be seen from individualsolution processed CNTs. Hyperspectral imaging of the tubes identifieslocal emission sites and shows that some previously dark quenchingsegments can be brightened by the application of high magnetic fields,and their effect on exciton transport and dynamics can be studied.Using focused high intensity laser irradiation, we introduce a singledefect into an individual nanotube which reduces its quantum efficiencyby the creation of a shallow bound exciton state with enhanced electron–holeexchange interaction. The emission intensity of the nanotube is thenreactivated by the application of the high magnetic field. [ABSTRACT FROM AUTHOR]

We report a comprehensive study ofthe tuning with electric fieldsof the resonant magneto-exciton optical phonon coupling in gated graphene.For magnetic fields around B∼ 25 T that correspondto the range of the fundamental magneto-phonon resonance, the electron–phononcoupling can be switched on and off by tuning the position of theFermi level in order to Pauli block the two fundamental inter-Landaulevel excitations. The effects of such a profound change in the electronicexcitation spectrum are traced through investigations of the opticalphonon response in polarization resolved magneto-Raman scatteringexperiments. We report on the observation of a splitting of the phononfeature with satellite peaks developing at particular values of theLandau level filling factor on the low or on the high energy sideof the phonon, depending on the relative energy of the discrete electronicexcitation and of the optical phonon. Shifts of the phonon energyas large as ±60 cm–1are observed close tothe resonance. The intraband electronic excitation, the cyclotronresonance, is shown to play a relevant role in the observed spectralevolution of the phonon response. [ABSTRACT FROM AUTHOR]

Positive trions in a quasi-two-dimensional hole gas confined in symmetric and asymmetric GaAs quantum wells are studied by a combination of polarization-resolved photoluminescence and transport measurements in high magnetic fields B (up to 23 T) and low temperatures (down to 30 mK). The experiments are accompanied by realistic numerical calculations. The whole family of trions (the singlet and a pair of triplets) are observed. The singlet-triplet crossing of Coulomb energies is found: hidden in symmetric and visible in asymmetric structures. [ABSTRACT FROM AUTHOR]

Abstract: Graphene multilayers are grown epitaxially on single crystal silicon carbide. This system is composed of several graphene layers of which the first layer is electron doped due to the built-in electric field and the other layers are essentially undoped. Unlike graphite the charge carriers show Dirac particle properties (i.e. an anomalous Berry’s phase, weak anti-localization and square root field dependence of the Landau level energies). Epitaxial graphene shows quasi-ballistic transport and long coherence lengths; properties that may persist above cryogenic temperatures. Paradoxically, in contrast to exfoliated graphene, the quantum Hall effect is not observed in high-mobility epitaxial graphene. It appears that the effect is suppressed due to the absence of localized states in the bulk of the material. Epitaxial graphene can be patterned using standard lithography methods and characterized using a wide array of techniques. These favorable features indicate that interconnected room temperature ballistic devices may be feasible for low-dissipation high-speed nanoelectronics. [Copyright &y& Elsevier]

Quantum dots with strong three dimensional confinement and low surface density have been identified in a structure which was nominally grown as a type-II GaAs/AlAs bilayer surrounded by GaAlAs barriers. Micro-luminescence experiments in magnetic fields performed on these dots display excitonic spin-splitting and orbital Zeeman effects for the excited states. The modification by the magnetic field of the diffusion and/or trapping of photoexcited carriers into the dots is also observed. [ABSTRACT FROM AUTHOR]

Recombination spectrum of excitons and positive trions is studied by two-beam magneto-photoluminescence of a two-dimensional hole gas. For acceptor-bound trions a low-energy cyclotron replica is observed, corresponding to a hole shake-up process. The experiment is supplemented by realistic numerical calculations, allowing for identification of individual transitions and connecting the splitting of the shake-up line directly with the hole mass. [ABSTRACT FROM AUTHOR]

Ensembles of sharp emission lines present in the macro-luminescence of type-II GaAa/AlAs double quantum well structures were studied. Micro-luminescence experiments allowed us to conclude that the sharp emission lines originate from lateral GaAlAs islands of a few mu;m in diameter, formed in the structure. They serve as efficient type-I recombination centers for indirect excitons and/or carriers diffusing in the GaAs/AlAs QW structure. Magneto-luminescence spectra from single islands resemble those observed for natural quantum dots formed in narrow GaAs quantum wells. The observed emission is assigned to the recombination of neutral excitons as well as excitonic molecules. [ABSTRACT FROM AUTHOR]

HgTe/CdHgTe quantum well (QW) heterostructures have attracted a lot of interest recently due to insights they provided towards the physics of topological insulators and massless Dirac fermions. Our work focuses on HgCdTe QWs with the energy spectrum close to the graphene-like relativistic dispersion that is supposed to suppress the non-radiative Auger recombination. We combine various methods such as photoconductivity, photoluminescence and magneto-optical measurements as well as transmission electron microscopy to retrofit growth parameters in multi-QW waveguide structures, designed for long wavelengths lasing in the range of 10–22 μm. The results reveal that the attainable operating temperatures and wavelengths are strongly dependent on Cd content in the QW, since it alters the dominating recombination mechanism of the carriers. [ABSTRACT FROM AUTHOR]

The effect of bis(trifluoromethane) sulfonimide (TFSI, superacid) treatment on the optical properties of MoS2 monolayers is investigated by means of photoluminescence, reflectance contrast and Raman scattering spectroscopy employed in a broad temperature range. It is shown that when applied multiple times, the treatment results in progressive quenching of the trion emission/absorption and in the redshift of the neutral exciton emission/absorption associated with both the A and B excitonic resonances. Based on this evolution, a trion complex related to the B exciton in monolayer MoS2 is unambiguously identified. A defect-related emission observed at low temperatures also disappears from the spectrum as a result of the treatment. Our observations are attributed to effective passivation of defects on the MoS2 monolayer surface. The passivation reduces the carrier density, which in turn affects the out-of-plane electric field in the sample. The observed tuning of the carrier concentration strongly influences also the Raman scattering in the MoS2 monolayer. An enhancement of Raman scattering at resonant excitation in the vicinity of the A neutral exciton is clearly seen for both the out-of-plane A′1 and in-plane E′ modes. On the contrary, when the excitation is in resonance with a corresponding trion, the Raman scattering features become hardly visible. These results confirm the role of the excitonic charge state plays in the resonance effect of the excitation energy on the Raman scattering in transition metal dichalcogenides. [ABSTRACT FROM AUTHOR]